That "constant power above base speed" statement needs to be expanded on a bit.
What happens is that your motor voltage stays constant as you move above the base speed (usually 50 or 60 Hz). So your V/Hz figure goes down as frequency goes up.
That doesn't immediately mean that your torque drops. But, the maximum available torque drops. And not inversely proportional to frequency, but inversely proportional to frequency squared.
This has two implications.
First, the slip increases, so the rotor gets hotter. But cooling increases, too. So you can still usually get the same torque even if you increase speed about ten percent. It all depends on what rotor you have.
Second, when frequency is increased so that peak torque comes down to the load line, the motor just stalls. Nothing more to do.
The range between rotor getting too hot (at 10 perhaps 20 percent overspeed) and when motor stalls is a grey zone where anything can happen. Best bet is that life of motor gets reduced due to high temperature.
This is for a fan with torque proportional to speed squared. Other loads and motors with more spare torque behave otherwise.
Gunnar Englund